Conventionally, as a method for nondestructively detecting a flaw that is present in a test object such as a steel plate and a steel pipe, a magnetic testing method (magnetic flux leakage testing method) has been known. This magnetic testing method is a flaw detecting method that uses the fact that, when a test object consisting of a magnetic substance is magnetized by applying a magnetic field, the magnetic flux leaks to a surface space in a portion including a flaw, if any, which blocks a magnetic flux produced on the test object.
In the above-described magnetic testing method, in order to increase the leakage flux leaking from the flaw to a detectable level, it is necessary to magnetize the test object to such a degree that the test object becomes magnetically saturated. Generally, as a magnetizing device for applying the magnetic field to the test object, an electromagnet, a coil, or the like of direct current or alternating current is used, and as a detecting device for detecting a leakage flux leaking from the flaw, a Hall element, a search coil, or the like is used.
As an apparatus for magnetically saturating the test object efficiently by using the magnetizing device such as an electromagnet, a coil, or the like, for example, the apparatuses described in Patent Literatures 1 and 2 have been proposed.
For the apparatus described in Patent Literature 1, a brush-form yoke is provided or a movable auxiliary yoke is provided between a magnetic pole (yoke open end) and a test object (material to be tested), whereby the occurrence of leakage flux caused by a gap between the magnetic pole and the test object is restrained, thereby improving the magnetization efficiency.
Unfortunately, for the apparatus described in Patent Literature 1, in the case where a DC electromagnet is used, the test object must be magnetically saturated in the whole thickness direction since the skin effect is not expected. In other words, there arises a problem that since a magnetic pole cross-sectional area larger than the cross-sectional area of the test object in the whole thickness direction is necessary, a large magnetizing device is required.
Hereunder, this apparatus is explained more specifically. The magnetic property of a ferromagnetic material constituting the test object such as a steel plate or a steel pipe has a nonlinear characteristic generally represented by a hysteresis curve. Therefore, magnetizing the test object up to about 1.4 T in terms of the magnetic flux density in the test object can be achieved by applying a relatively weak magnetic field. However, in order to obtain a magnetic flux density near the saturated magnetic flux density (1.7 to 1.81 T for a general carbon steel) necessary for sufficiently obtaining the leakage flux leaking from the flaw, it is necessary to apply an extremely strong magnetic field to the test object. Further, in DC magnetic saturation, the magnetic flux is distributed uniformly in the thickness direction of the test object. Therefore, in order to magnetically saturate the test object by using a DC electromagnet, it is necessary to use a large-size magnetizing device depending on the size (thickness) of the test object.
To solve the above-described problem, as described in Patent Literature 2, a magnetizing device using an AC electromagnet is adopted, and only the outer layer of test object has only to be magnetized by utilizing the skin effect. According to the apparatus described in Patent Literature 2, the size of the magnetizing device can be decreased. However, as described in Patent Literature 2, in the case where the test object is magnetized by applying an AC magnetic field to such a degree that the object becomes magnetically saturated, there arises a problem that since a large amount of heat is generated by an eddy current produced in the test object, an adverse influence such as decreased sensitivity or decreased service life of detecting device for detecting leakage flux occurs.
Hereunder, this apparatus is explained more specifically. In the case where the AC magnetic field is applied, since the magnetic flux can be concentrated on the outer layer of test object by the skin effect, this apparatus has an advantage that the size of the magnetizing device can be decreased as compared with the case where the DC magnetic field is applied. However, in order to suppress a noise signal caused by the material quality of test object, it is necessary to increase the magnetic flux density in the test object to the vicinity of the saturated magnetic flux density as in the case where the DC magnetic field is applied. In the case where the test object is magnetized by the AC magnetic field only to such a degree that the object becomes magnetically saturated, an electromotive force proportional to the time change of magnetic flux develops in the test object, which results in the occurrence of an eddy current. The current flowing in the test object is accompanied by resistance heat generation, and the test object becomes in an induction heated state, which causes the change in temperature of the leakage flux detecting device and its attachment jig provided in the surroundings. Generally, as the leakage flux detecting device, a sensor such as a Hall element, a search coil, or a flux gate is used. In the case where either of these sensors is used, an influence is exerted on the detection sensitivity of leakage flux and the service life on account of the change in temperature.